A brushless direct-current (BLDC) motor uses semiconductor switches to accomplish electronic phase changes, and thus has advantages, such as less mechanical wear and lower noise, as compared to a motor using a mechanical rectifier that is established by carbon brushes and commutators.
As shown in FIG. 1, a driving circuit for a BLDC motor 10 includes an H-bridge circuit, which is established by four MOSFETs Q1-Q4 that act as switches and have body diodes D1-D4, respectively, and has two output terminals A and B to be connected to the BLDC motor 10, a pulse width modulation (PWM) controller 12, which has four output terminals AH, AL, BH, and BL to provide PWM signals to control the MOSFETs Q1-Q4, respectively, to thereby generate an operational voltage as required between the output terminals A and B to adjust the winding current Im of the BLDC motor 10 and accordingly the rotation speed of the BLDC motor 10, and an over-current protection (OCP) circuit 14 connected between the H-bridge circuit and a ground terminal to detect the winding current Im for providing the system an over-load protection. When the winding current Im is so large to indicate that the system is at an over-load state, the OCP circuit 14 will signal the PWM controller 12 to stop providing the PWM signals, or to directly turn off the lower-side MOSFETs Q2 and Q4, thereby stopping the BLDC motor 10.
For the sake of convenient illustration, the MOSFETs Q1-Q4 are called the first upper-side switch, the first lower-side switch, the second upper-side switch, and the second lower-side switch, respectively. The driving process of the BLDC motor 10 is shown in FIG. 2. For example, as shown in FIG. 2(a), when the rotor of the BLDC motor 10 is at one of the phases, the PWM controller 12 maintains the second upper-side switch Q3 off and the second lower-side switch Q4 on, and alternatively switches the first upper-side switch Q1 and the first lower-side switch Q2 by a PWM signal, so that the winding current Im flows from the output terminal A through the BLDC motor 10 to the output terminal B; when the rotor is at another phase, as shown in FIG. 2(b), the PWM controller 12 maintains the first upper-side switch Q1 off and the first lower-side switch Q2 on, and alternatively switches the second upper-side switch Q3 and the second lower-side switch Q4, so that the winding current Im flows from the output terminal B through the BLDC motor 10 to the output terminal A. For the sake of convenient illustration, the phase depicted in FIG. 2(a) is referred to as the first phase, and that depicted in FIG. 2(b) is referred to as the second phase. When the BLDC motor 10 is switched between the first and the second phases, if the switch timing is not properly controlled, a phase-change surge current will occur due to the residual current Im in the windings of the BLDC motor 10 and thereby induce a reactive electromotive force to boost the voltage at the power input terminal Vin, as shown in FIG. 3 for example, which may damage corresponding components.
In order to prevent the voltage Vin from instantly being boosted during a phase change, a dead time is inserted when the BLDC motor 10 is switched between different phases, for the winding current Im to decay to zero before the BLDC motor 10 is switched from the current phase to the next phase. For example, referring to FIG. 4, to switch from the first phase to the second phase, during the dead time inserted therebetween, the PWM controller 12 maintains the second lower-side switch Q4 on and the other switches Q1-Q3 off, thereby establishing a current loop to allow the winding current Im to be consumed by the second lower-side switch Q4 and the body diode D2, or maintains the first lower-side switch Q2 and the second lower-side switch Q4 on and the other switches Q1 and Q3 off, thereby establishing a current loop to allow the winding current Im to be consumed by the first lower-side switch Q2 and the second lower-side switch Q4. However, the winding current Im varies with the rotation speed of the BLDC motor 10 and thus requires different time periods to decay to zero at different rotation speeds, i.e., different rotation speeds require different dead times. For instance, if the dead time is such set that the residual current Im can be completely consumed at the rotation speed of 50%, then for the rotation speed of 70%, the residual current Im will be still high enough to cause a phase-change surge current after the dead time terminates. On the contrary, if the dead time is set longer, the maximum rotation speed of the BLDC motor 10 will be adversely limited.
Therefore, it is desired a method and apparatus for dynamically adjusting a dead time of a BLDC motor during a phase change.